This invention relates generally to a method of necking-in the open end of a cylindrical container and, more specifically, to a method of die-necking the open end of a container including a plurality of die-necking steps which form a smooth neck configuration on the open end of the can.
In recent years it has become commonplace in the beer and beverage industry to use two piece cans formed of thin sheet aluminum and consisting of a first cylindrical can body having an integral bottom end panel and an upper open end which, after filling, is closed by attaching a separate end cap onto the mouth of the open end of the can body. Because of the thickness of the metal used in forming the end cap, the cost of the cap is a very significant portion of the overall cost of the can. Consequently, a long standing cost reduction trend in the aluminum can industry has been to decrease the diameter of the mouth of the can by making a neck that is smaller than the diameter of the cylindrical can body in order to use a smaller end cap and thereby save on the cost of the metal for the end cap. For example, the outside diameter of the cylindrical body of a twelve ounce or sixteen ounce can is commonly 2-11/16 inches (a 211 diameter) and the open end of the can may be necked down to a diameter of 2-6/16 inches (a 206 diameter), with the continuing trend in the industry towards even smaller diameters, for example a diameter of 2-4/16 inches (a 204 diameter).
In the past, various processes including triple necking or quad necking operations have been employed to produce a stepped or ribbed neck having a reduced diameter of desired size. In addition, prior U.S. Pat. No. 3,029,507, 4,173,883, 4,403,493, 4,527,412, and 4,774,839 disclose various processes employing a plurality of die necking steps attempting to form smooth walled necks. However, as the diameter of the neck becomes smaller and smaller, it has become more difficult to provide a smooth neck profile free of wrinkles or pleats.
Also, a foreign subsidiary of the assignee of this invention for some time has employed a die-necking process in which the reduced diameter neck is formed by a plurality of die necking steps. The first step produces a reduced diameter neck corresponding in axial length to the desired axial length of the finished neck and each successive die necking step then reduces the diameter of the neck further while reforming by actual die contact only a portion of the length of the neck formed in the preceding step. This method is similar to the operations described in the above-noted patents
In die-necking processes the first necking step is the most critical and it is in that step where wrinkles or pleats are most commonly formed which produce scrap containers. Saunders U.S. Pat. Nos. 3,964,413 and 3,995,572 propose to avoid wrinkling problems by providing die-necking methods in which the first step forms a strengthening hoop at the peripheral edge of the can and a subsequent die necking step or steps form the final full length neck. However, the hoop formed in each of these systems is not totally reformed by die contact in the subsequent step and the final configuration of the neck remains directly dependent on the configuration of the hoop.
The invention described herein is directed to the elimination of the wrinkling or pleating problems associated with prior, multiple step, die-necking operations, particularly those associated with producing smooth walled 206, 204, and smaller diameter necks.